The present invention relates to electromotive devices including electromagnetically excited machines and permanent magnet synchronous machines (PMM's). For clarity and simplicity, the term permanent magnet synchronous machine (PMM) is used throughout this specification, but it should be clearly understood that the term is intended to include electromotive devices in general, including electromagnetically excited machines.
The electromagnetic forces developed in a PMM result from the interaction of a permanent magnet field with a stator current. This interaction can be fundamentally represented by the relation:F=k*B*K*S Where:                F is the electromagnetic force        k is a proportionality constant depending on geometry,        B is the airgap flux density,        K is the armature surface current density, and        S is the airgap surface area.        
In order to increase the power density of the PMM, it is necessary to increase the airgap flux density B or the armature surface current density K, or the airgap surface area S (however increasing the airgap surface area typically implies making a larger machine, which may well increase the power but not the power density of the device). The airgap flux density B is limited by the magnetic properties of the permanent magnets and the saturation properties of any flux-carrying components. The maximum current density K is ultimately limited by the conductor insulation temperature rating and the thermal demagnetization of the permanent magnets. These temperatures, in turn, depend on the machine's inherent thermal characteristics and cooling mechanism. Most approaches to increasing power density use aggressive cooling methods to allow more current for a given temperature rise.